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| United States Patent |
5,134,356
|
|
El-Sharkawi
, et al.
|
July 28, 1992
|
Reactive power compensator
Abstract
A system and method for determining and providing reactive power
compensation for an inductive load. A reactive power compensator (50,50')
monitors the voltage and current flowing through each of three
distribution lines (52a, 52b, 52c), which are supplying three-phase power
to one or more inductive loads. Using signals indicative of the current on
each of these lines when the voltage waveform on the line crosses zero,
the reactive power compensator determines a reactive power compensator
capacitance that must be connected to the lines to maintain a desired VAR
level, power factor, or line voltage. Alternatively, an operator can
manually select a specific capacitance for connection to each line, or the
capacitance can be selected based on a time schedule. The reactive power
compensator produces control signals, which are coupled through optical
fibers (102/106) to a switch driver (110, 110') to select specific
compensation capacitors (112) for connections to each line. The switch
driver develops triggering signals that are supplied to a plurality of
series-connected solid state switches (350), which control charge current
in one direction in respect to ground for each compensation capacitor.
During each cycle, current flows from ground to charge the capacitors as
the voltage on the line begins to go negative from its positive peak
value. The triggering signals are applied to gate the solid state switches
into a conducting state when the potential on the lines and on the
capacitors reaches a negative peak value, thereby minimizing both the
potential difference and across the charge current through the switches
when they begin to conduct. Any harmonic distortion on the potential and
current carried by the lines is filtered out from the current and
potential signals used by the reactive power compensator so that it does
not affect the determination of the required reactive compensation.
| Inventors:
|
El-Sharkawi; Mohamed A. (Renton, WA);
Venkata; Subrahmanyam S. (Woodinville, WA);
Chen; Mingliang (Kirkland, WA);
Andexler; George (Everett, WA);
Huang; Tony (Seattle, WA)
|
| Assignee:
|
Board of Regents of the University of Washington (Seattle, WA)
|
| Appl. No.:
|
665774 |
| Filed:
|
March 7, 1991 |
| Current U.S. Class: |
323/211; 323/209 |
| Intern'l Class: |
G05F 001/70 |
| Field of Search: |
323/208,209,210,211,218
|
References Cited
U.S. Patent Documents
| 4263646 | Apr., 1981 | Beatty | 363/58.
|
| 4266140 | May., 1981 | Kaufman | 250/551.
|
| 4370607 | Jan., 1983 | Dassonville | 323/271.
|
| 4371830 | Feb., 1983 | Loucks | 323/265.
|
| 4516037 | May., 1985 | Shackle | 307/252.
|
| 4586105 | Apr., 1986 | Lippmann et al. | 361/117.
|
| 4611123 | Sep., 1986 | McDonald | 250/551.
|
| 4645997 | Feb., 1987 | Whited | 323/211.
|
| 4677364 | Jun., 1987 | Williams et al. | 323/211.
|
| 4704543 | Nov., 1987 | Barker et al. | 307/106.
|
Primary Examiner: Stephan; Steven L.
Assistant Examiner: Voeltz; Emanual Todd
Attorney, Agent or Firm: Christensen, O'Connor, Johnson & Kindness
Goverment Interests
This invention was made with government support under grant number DE
AI79-87BP65584, awarded by the Department of Energy. The government has
certain rights in the invention.
Parent Case Text
This application is a continuation-in-part, of pending prior application
Ser. No. 07/543,118, filed on June 22, 1990, now abandoned. The benefit of
the filing date of which is hereby claimed under 35 U.S.C. .sctn.120.
BACKGROUND OF THE INVENTION
A common problem on electrical power distribution systems supplying power
to inductive loads is the need to provide reactive power compensation.
Large motors and other types of inductive loads used, for example, in
lumber mills, rock crushing plants, steel mills, and to drive elevators
and pumps, shift the power factor of the system away from the desired
unity level, thereby decreasing the efficiency of the power system.
Compensation for the effects of inductive loads can be provided to control
line voltage, power factor, or volt-ampere-reactive (VAR) power. Such
compensation generally takes the form of capacitor banks that are
connected to transmission and distribution lines. While an appropriate
capacitive compensation can be determined and left on-line to compensate
continuously running inductive loads, most inductive loads operate
intermittently and cyclically, requiring that the correct compensation be
selectively applied in response to a varying reactive load on the system.
Mechanical contactors are typically employed to connect and switch the
capacitor banks to compensate changing inductive loads. However,
mechanical contractors are known to introduce undesirable transients each
time that they operate to change the reactive compensation. Furthermore,
being mechanical devices, mechanical contractor must be maintained,
rebuilt, or even replaced after a limited number of operating
Other devices have been developed for controlling reactive power at the
point of use, such as a power factor control system for induction motors
developed by Frank J. Nola, which is described in U.S. Pat. No. 4,266,177.
Unfortunately, there are several problems with the Nola control system
that have prevented it from being widely used. For example, operational
parameters of this type of device must be tailored for use with a specific
inductive load. In addition, the Nola device can control power factor over
only a limited range restricted to light loads, since the device will not
work at full loads. Three Nola devices are required to control the power
factor of a three-phase inductive load.
A reportedly transient-free, solid state automatic power factor correction
apparatus is disclosed in U.S. Pat. No. 4,645,997. This apparatus is
designed to automatically correct power factor in a multi-phase system, on
the load side of a distribution transformer, e.g., to correct the power
factor of an inductive load within a plant. It generates signals
indicative of the voltage and current associated with each phase supplying
power to the load. The current and voltage signals for each phase are
compared to each other to determine the extent of current lag, and a
signal indicative of current lag is generated for each line. A
microprocessor-controlled circuit converts these signals into a lagging
phase angle in degrees and determines the cosine of the angle and thus,
the power factor of the line. The microprocessor also controls a switching
network that is capable of selectively adding or removing banks of
delta-connected capacitors to or from the power lines to control power
factor. Only two-silicon-controlled rectifiers (SCRs) comprise the
switching network for each bank of capacitors. According to this patent,
the SCRs can connect the capacitor banks to the lines at any time,
regardless of the voltage on the capacitors, without creating current
surges or electrical transients. However, this device is intended to
operate at relatively low distribution transformer secondary voltage
levels typically used in a plant, i.e., 480 volts or less, and it cannot
accurately determine the required reactive power compensation to control
power factor if there are significant harmonics of the fundamental 60 Hz
line frequency present in the current or voltage, since such harmonic
distortion interferes with the measurement of the phase angle or lag time
between voltage and current. In addition, the system disclosed in U.S.
Pat. No. 4,645,997 lacks other features required for fully automated,
unattended operation, such as the capability for remote control of the
switching network and the ability to detect and compensate for
malfunctions in the device. In any case, it is generally more effective
for an electric utility to provide reactive power compensation on the
lines of an electric power distribution system rather than depending on
the customer to correct each load. The device disclosed in this patent
cannot be used on distribution lines, because it cannot be controlled
remotely, cannot operate unattended, and cannot operate at the higher
voltages typically used on distribution lines (up to 35 KV).
In U.S. Pat. No. 4,645,364, which is issued to Williams and two of the
inventors of the present invention, a reactive power compensating system
is disclosed that is designed to directly compensate an inductive load on
multi-phase lines of a distribution system. The apparatus includes fixed
capacitors that are always connected to each phase of the system to
provide a minimum reactive power compensation, and selectively switched
capacitors that are connected to each phase by a solid state switching
network of SCR and diode pairs to provide any additional compensation
required. The required reactive power compensation for all phases is
determined in the device by sensing the current on only one phase at the
time its voltage crosses zero.
The reactive power compensation system described in the Williams et al.
patent is deficient in several important respects. Since only one phase of
a multi-phase distribution system is used to determine the required
reactive power compensation for all of the phases, the system cannot
properly compensate for different inductive demands on each of the phases
resulting, for example, from various single phase and multi-phase
inductive motors. Also, the determination of the required reactive power
compensation is susceptible to errors caused by harmonic distortion in the
line current and voltage--a problem that is specifically admitted in the
patent. In some situations, use of a fixed capacitance to provide a
minimum required compensation may be inappropriate, since, if all of the
significant inductive loads connected to the system are at time
de-energized, the correct compensation may be much less than that provided
by the fixed capacitance. Furthermore, fault tolerance and operation of
the switching network in the presence of voltage transients and harmonics
are not addressed in this reference.
In consideration of the above-noted problems that exist with the prior art
systems used to compensate for inductive loads, it is an object of the
present invention to provide an appropriate reactive power compensation
for each phase of a multi-phase system. It is desirable that this reactive
power compensator be selectively operable to maintain either a specified
power factor, voltage level, or VAR level as the reactive load changes, on
a phase-by-phase basis. The reactive power compensator should react in
less than one cycle to changes in the required power compensator should
react in less than one cycle to changes in the required reactive power
compensation, by selectively connecting the appropriate capacitance to
provide that compensation, even in the presence of harmonic distortion in
the voltage or current, without itself introducing any transients or
distortion. Further, the reactive power compensator should be unaffected
by variations in line frequency, and should provide fault-tolerant
operation. The ability to control the reactive power compensator remotely
is also desirable so that it can be readily integrated into a
centrally-controlled and automated power distribution system. In addition,
it is desirable that the device be selectively operable in a manual or
automatic mode. These and other objects and advantages of the present
invention will be apparent from the attached drawing and from the
Description of the Preferred Embodiments that follows.
SUMMARY OF THE INVENTION
The present invention is a reactive power compensating system for
compensating a reactive load that draws power from different phases of a
supply. The system includes a plurality of electrical current sensors,
each of which is connected to sense a generally periodically varying
electrical current supplied to the reactive load independently on each of
the phases, producing a current signal corresponding to the electrical
current. Similarly, a plurality of voltage sensors are included, each of
which is connected to sense a generally periodically varying voltage on
one of the phases, producing a potential signal corresponding to the
voltage. Reactive power sensing means are connected to receive the current
signal and the potential signal for each phase and are operative to
determine a required reactive power compensation for each phase,
appropriate to separately compensate each phase for the reactive load
connected thereto. Significantly, the required reactive power compensation
is accurately determined even if harmonic distortion exists in the current
and/or voltage on the phase.
The reactive power compensation is provided by one or more of a plurality
of capacitors. The plurality of capacitors are preferably assembled in
arrays, each of which is associated with one of the phases. Each of the
capacitors in an array is selectively connectable to provide the reactive
power compensation required by the phase associated with that array.
Switching means, which are electrically connected to the plurality of
capacitors and to the reactive power sensing means, selectively connect
the capacitors to provide the reactive power compensation determined for
each phase by the reactive power sensing means. The reactive power
compensation provided for each phase can thus be different.
Preferably, the reactive power sensing means comprise processor means that
determine the reactive power compensation required for each phase as a
function of the current signal at a predetermined point in a cycle of the
potential signal for the phase. The processor means also control the
switching means, causing the switching means to provide the required
reactive power compensation determined for each phase at a subsequent time
in the cycle of the potential signal. The reactive power compensation
required for each phase can be determined as often as twice each cycle of
the potential signal, and the switching means are operative to connect the
capacitors required to provide the reactive power compensation, which can
be determined as often as twice each cycle.
The processor means are selectively operable in a manual mode, an automatic
mode, and a time control mode. Furthermore, the processor means are
selectively operable to determine and control either a VAR level, the
voltage, or a power factor value on each phase. The reactive power sensing
means comprise filter means that are connected to receive the current
signal and the potential signal for each phase. Harmonics of the
fundamental frequency on these signals are filtered out by the filter
means, respectively producing a filtered current signal and a filtered
potential signal.
In one preferred form of the invention, the voltage sensor comprises a
plurality of transformers having primary windings and secondary windings,
each of the primary windings being connected to receive a potential
corresponding to the voltage on one of the phases. The secondary windings
provide the potential signals for the phases so that a phasal relationship
between the potential signals defines an interval prior to the
predetermined point in the cycle of the potential signal during which the
current signal is sampled. In another form of the invention, the voltage
sensor comprises a plurality of voltage transformers, each connected to a
different phase and operative to produce the potential signal for that
phase. The processor means are then operative to determine the
predetermined point in the potential signal and the subsequent time for
control of the switching means as a function of the potential signal.
The switching means comprise a plurality of solid state switches connected
in a plurality of series strings. Each series string is activated in
response to a switching signal to connect one of the capacitors to provide
at least part of the reactive power compensation required for one of the
phases. The reactive power sensing means provide the switching signal to
the solid state switches when the potential signal for the phase is at a
level such that substantially no electrical current flows through the
solid state switches at the instant they are activated. The switching
means preferably include oscillator means for producing pulses at a
frequency substantially higher than that of a fundamental frequency of the
voltage on the phases. The switching signal comprises an envelope of the
pulses at the higher frequency that is generated during a predetermined
portion of the cycle occurring at about the peak of the potential signal.
The pulses repetitively turn on the solid state switches so that
distortion in the voltage on the line does not cause the solid state
switches to turn off prematurely.
Feedback means are provided for sensing the reactive power compensation
actually provided by the capacitors and for producing a feedback signal
for each capacitor, which indicates the reactive power compensation that
it is providing. These feedback signals are used by the reactive power
sensing means to determine if the switching means have malfunctioned. If a
malfunction is thus detected, an alternate configuration of capacitors is
effected to provide a best available reactive power compensation. Means
are also provided to protect each of the phases against an over voltage
condition and against self-excitation by an inductive load, in respect to
the reactive power compensation provided by the system.
A method of compensating for a reactive load that draws power from a
multi-phase supply is another aspect of this invention. The method
includes steps generally consistent with the functions provided by the
elements comprising the system described above.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A reactive power compensating system, comprising:
a. an electrical current sensor that produces a current signal
corresponding to an instantaneous value of an electrical current supplied
through a line to an inductive load, the electrical current generally
varying periodically at a fundamental frequency;
b. a voltage sensor that produces a potential signal corresponding to an
instantaneous voltage on the line, the potential signal varying according
to a generally periodic cycle at the fundamental frequency;
c. filter means, connected to receive the current signal, for producing a
filtered current signal from which any harmonics of the fundamental
frequency that are present in the current signal are substantially
filtered out;
d. processor means, connected to receive the potential signal and the
filtered current signal, for determining a reactive power compensation for
the inductive load as a function of the filtered current signal when the
potential signal is at a predefined point in its periodic cycle and
operative to produce a switching signal to select that reactive power
compensation;
e. a plurality of capacitors, each selectively connectable to compensates
reactance on the line; and
f. switching means, connected to the plurality of capacitors and connected
to receive the switching signal from the processor means, for selectively
connecting the capacitors to provide the reactive power compensation
determined by the processor means in response to the switching signal, the
reactive power compensation being substantially free of errors that would
otherwise be caused by the harmonics of the fundamental frequency present
in the electrical current in the line.
2. The reactive power compensation system of claim 1, wherein the processor
means are selectively operated in one of a manual mode, an automatic mode,
and a time control mode.
3. The reactive power compensation system of claim 2, wherein while
operating in the automatic mode, the processor means are operative to
provide reactive power compensation so as to selectively control reactive
volt amperes, the voltage of the line, or a power factor associated with
the line.
4. The reactive power compensation system of claim 1, wherein the switching
means are operative to connect the capacitors to provide the reactive
power compensation only when an absolute value of the voltage on the line
is at a maximum, as often as twice during each periodic cycle of the
voltage.
5. The reactive power compensation system of claim 1, wherein the switching
means are operative to connect the capacitors to provide the reactive
power compensation only when an electrical current flow through the
switching means is initially substantially zero as the switching means
connect the capacitors.
6. The reactive power compensation system of claim 1, wherein the filter
means are connected to receive the potential signal and are further
operative to substantially filter out harmonics of the fundamental
frequency that are present from the potential signal, producing a filtered
potential signal.
7. The reactive power compensation system of claim 1, wherein the processor
means include means for sampling the filtered current signal for a
predetermined portion of the periodic cycle prior to the potential signal
reaching its redefined point in the periodic cycle.
8. The reactive power compensation system of claim 7, wherein the
predefined point in the periodic cycle occurs when the voltage on the line
is equal to zero, the processor means being operative to sample and hold
the filtered current signal at that instant to determine the reactive
power compensation.
9. The reactive power compensation system of claim 1, further comprising
feedback means for monitoring the reactive power compensation provided by
the capacitors and producing a feedback signal indicative thereof that is
supplied to the processor means.
10. The reactive power compensation system of claim 9, wherein the feedback
means comprise a plurality of current transformers, each of which is
connected to measure a reactive power compensation current provided by one
of the capacitors, and in response thereto, producing the feedback signal,
the processor means using the feedback signal to determine if the reactive
power compensation being supplied by the capacitors is substantially equal
to the required reactive power compensation determined by the processor
means.
11. The reactive power compensation system of claim 10, wherein the
processor means detects a malfunction in the switching means or one of the
capacitors in response to the feedback signal failing to match an expected
reactive power compensation, and upon detecting a malfunction, selectively
causes connection of an alternative capacitor to the line to provide the
reactive power compensation using a different configuration of capacitors
than used before the malfunction.
12. The reactive power compensation system of claim 10, wherein in response
to the potential signal, the processor means protects against an over
voltage on the line.
13. The reactive power compensation system of claim 1, further comprising
means for extending the time during which the switching signal is provided
so that voltage variations on the line do not cause a premature
interruption of a charge current flowing to the capacitors through the
switching means.
14. The reactive power compensation system of claim 13, wherein the means
for extending the time comprise means for producing an envelope of pulses,
the pulses occurring at a frequency substantially higher than the
fundamental frequency of voltage on the line.
15. A reactive power compensating system for compensating a reactive load
drawing power from multiple phases of a supply, comprising:
a. a plurality of electrical current sensors, each of which is connected to
sense a generally periodically varying electrical current supplied to the
reactive load on a different one of the phases, producing a current signal
corresponding to said electrical current;
b. a voltage sensor connected to sense a generally periodically varying
voltage on each of the phases, producing a plurality of potential signals,
each potential signal corresponding to the voltage on one of the phases;
c. reactive power sensing means, connected to receive the current signal
and the potential signal for each phase, for determining a separate
reactive power compensation for each phase, each of which can be
different, as required to compensate each phase for the reactive load
connected to that phase, and further operative to produce switching
signals to select the reactive power compensation for each phase;
d. a plurality of capacitors assembled in arrays, each array being
associated with one of the phases, each of the capacitors in the arrays
being selectively connectable to provide at least a portion of the
reactive power compensation required for one of the phases; and
e. switching means, electrically connected to the plurality of capacitors
and connected to receive the switching signals for the reactive power
sensing means, for selectively connecting the capacitors to provide the
separate reactive power compensation independently determined for each
phase by the reactive power sensing means, in response to the switching
signals.
16. The reactive power compensating system of claim 15, wherein the
reactive power sensing means comprise processor means for determining the
reactive power compensation for each phase as a function of the current
signal at a predetermined point in a cycle of the potential signal and for
controlling the switching means using the switching signals to provide the
required reactive power compensation determined for each phase at a
subsequent time in the cycle.
17. The reactive power compensating system of claim 16, wherein for each
phase, the processor means are operative to determine the reactive power
compensation as often as twice each cycle of the potential signal, and
operative to cause the switching means to connect the capacitors required
to provide the reactive power compensation determined as often as twice
each cycle.
18. The reactive power compensating system of claim 16, wherein the
processor means are selectively operable in one of a manual mode, an
automatic mode, and a time control mode.
19. The reactive power compensating system of claim 16, wherein the
processor means are selectively operable to determine and to control on
each phase one of:
a. a volt-ampere-reactive level;
b. the voltage; and
c. a power factor associated with the phase.
20. The reactive power compensating system of claim 15, wherein the
reactive power sensing means comprise filter means, connected to receive
the current signal and the potential signal for each phase, for producing
a filtered current signal and a filtered potential signal for each phase
from which any harmonics of a fundamental frequency are substantially
filtered out.
21. The reactive power compensating system of claim 15, wherein the voltage
sensor comprises a plurality of transformer primary windings, each of the
primary windings being connected to receive a potential corresponding to
the voltage on one of the phases, and secondary windings, the secondary
windings providing the potential signals for the phases, so that a phasal
relationship between the potential signals defines an interval prior to
the predetermined point in the cycle of the potential signal during which
the current signal is sampled.
22. The reactive power compensating system of claim 16, wherein the voltage
sensor comprises a plurality of voltage transformers, each connected to
one of the phases and operative to produce one of the potential signals,
and wherein the processor means are operative to determine the
predetermined point and the subsequent time in the cycle for each phase as
a function of both the potential signal and a predefined time interval.
23. The reactive power compensating system of claim 15, wherein the
switching means comprise a plurality of solid state switches connected in
a plurality of series strings, the series string being activated in
response to the switching signal to connect one of the capacitors to
provide at least part of the reactive power compensation required for one
of the phases.
24. The reactive power compensating system of claim 23, wherein the
reactive power sensing means provide the switching signal to the solid
state switches to effect the reactive power compensation required for one
of the phases only when an initial electrical current through the solid
state switches is substantially zero as the solid state switches are
activated.
25. The reactive power compensating system of claim 23, wherein the
switching means further comprise oscillator means for providing pulses at
a frequency substantially higher than that of a fundamental frequency of
the voltage on the phases, and wherein the switching signal is in the form
of an envelope of the pulses, the pulses in the envelop repetitively
gating the solid state switches so that any distortion in the voltage does
not cause a premature interruption of a charge current flowing through the
solid state switches to the capacitors.
26. The reactive power compensating system of claim 15, further comprising
feedback means for sensing the reactive power compensation actually
provided by each capacitor and producing feedback signals indicative
thereof used to verify proper operation of the system.
27. The reactive power compensating system of claim 26, wherein the
reactive power sensing means use the feedback signals to determine if the
switching means have malfunctioned, and if so, effect a best available
alternative configuration of the capacitors to provide the reactive power
compensation.
28. The reactive power compensating system of claim 15, wherein the
reactive power sensing means further comprise means for protecting each of
the phases against an over voltage condition and against self-excitation
by an inductive load, in respect to the reactive power compensation
provided by the system.
29. A system for compensating reactive power loads in an alternating
current power system having at least one phase, comprising:
a. voltage monitoring means for monitoring the voltage in each phase of the
power system;
b. synchronization means for synchronizing to each phase in the power
system without regard for any harmonics of a fundamental frequency
present, and producing at least one synchronization signal in respect to
each of the phases;
c. current monitoring means for monitoring the current in each phase of the
power system;
d. triggering means for producing triggering signals, each triggering
signal being used to select a specific reactive power compensation;
e. processor means, connected to the voltage monitoring means, the current
monitoring means, and the synchronization means for:
i. determining a reactive current in each phase when the voltage in that
phase is at a predetermined synchronization level;
ii. determining a reactive power compensation for each phase, based on the
reactive current determined in (i); and
iii. controlling production of the triggering signals by the triggering
means; and
f. compensation means, responsive to the trigger signals, for applying the
reactive power compensation selected by the trigger signals, as determined
by the processor means, independently to each phase.
30. The system of claim 29, further comprising means for filtering out
harmonic distortion of the current.
31. The system of claim 29, further comprising means for filtering out
harmonic distortion of the voltage.
32. The system of claim 29, wherein the synchronization level is
independent of any frequency drift of the voltage on the phases.
33. The system of claim 29, wherein the processor means are selectively
operable in a manual mode, an automatic mode, or a time control mode.
34. The system of claim 29, wherein the synchronization means comprise a
circuit that produces the synchronization signals in response to a phasal
relationship between the voltages on the phases, the synchronization
signals being used in determining both the reactive power compensation
required for each phase and a time when that reactive power compensation
is connected to each phase.
35. The system of claim 29, wherein the synchronization means comprise:
a. a plurality of voltage transformers, each connected to one of the phases
and operative to produce a potential signal;
b. filter means for filtering out substantially all harmonics of the
fundamental frequency, producing a filtered potential signal for each
phase; and
c. a zero crossing detector that produces the synchronization signal for
each phase as a function of the filtered potential signal.
36. The system of claim 29, wherein the processor means comprise a sample
and hold circuit that responds to the synchronization means to being
sampling the current in each phase upon the occurrence of one
synchronization signal and holds the current sampled when another
synchronization signal for that phase occurs.
37. The system of claim 29, wherein the compensation means comprise a
plurality of capacitors that are selectively connected to the phase to
provide the reactive power compensation determined by the processor means.
38. The system of claim 37, further comprising means for monitoring
operation of the compensation means, producing a feedback signal
indicating the extent of the reactive power compensation being provided by
the compensation means.
39. The system of claim 29, wherein the switching means are operative to
protect against a partial failure of the switching means by reconfiguring
the compensation means to provide a best available reactive power
compensation.
40. The system of claim 29, wherein the switching means are operative to
connect the compensation means to the phases in response to the triggering
signal indicating that the voltage on the phases has reached an absolute
peak value, whereby an electrical current flow through the switching means
as the compensation means are connected is initially substantially zero.
41. The system of claim 40, wherein the switching means include means to
repetitively retrigger the switching means, so that variations in the
voltage on the phase do not affect provision of the compensation.
42. An adaptive reactive power compensation system for compensating
reactive power loads in an alternating current power system having at
least one phase, comprising:
a. monitoring means for monitoring the voltage in each phase of the power
system;
b. synchronization means for synchronizing to the voltage on each phase in
the power system, producing a synchronization signal in respect to each of
the phases;
c. current monitoring means for monitoring the current in each phase of the
power system;
d. triggering means for producing a triggering signal for each phase;
e. processor means, connected to the voltage monitoring means, the current
monitoring means, and the synchronization means for:
i. determining a reactive current in each phase when the voltage on that
phase is at a predetermined synchronization level;
ii. determining a desired reactive power compensation for each phase a
function of the reactive current; and
ii. controlling production of the triggering signal by the triggering means
to effect the desired reactive power compensation;
f. a plurality of capacitors, each selectively connected to provide
reactive power compensation to one of the phases; and
g. a plurality of switching blocks, each of which is associated with one of
the phases, each switching block being connected to the triggering means
and responsive to the triggering signal so that an appropriate capacitance
can be connected by the switching block to each phase to provide the
desired reactive power compensation determined by the processor means, the
triggering signal enabling said capacitance to be connected at an
appropriate time, each capacitor including means for sampling a charge
current flowing to the capacitor and producing a closed loop feedback
signal indicative thereof, the processor means using the closed loop
feedback signal to verify that the appropriate reactive power compensation
is being supplied to each phase.
43. The system of claim 42, wherein the processor means are operative to
detect a failure of a component in the system if the closed loop feedback
signal differs from an expected value.
44. A method of compensating reactive power, comprising the steps of:
a. producing a current signal corresponding to an electrical current
supplied through a line to an inductive load, the electrical current
generally varying periodically at a fundamental frequency;
b. sensing a voltage on the line to produce a potential signal that varies
in a generally periodic cycle at the fundamental frequency;
c. filtering the current signal to produce a filtered current signal from
which any harmonics of the fundamental frequency that are present in the
current signal are substantially reduced;
d. determining a reactive power compensation for the inductive load based
on the filtered current signal when the potential signal is at a
predefined point in its periodic cycle; and
e. selectively connecting an appropriate capacitance to provide the
reactive power compensation determined for the line, the reactive power
compensation being substantially free of an error that would otherwise be
caused by the harmonics of the fundamental frequency present in the
electrical current in the line.
45. The method of claim 44, further comprising the step of selectively
compensating reactive power in one of a manual mode, an automatic mode,
and a time control mode.
46. The method of claim 45, wherein while operating in the automatic mode,
the reactive power compensation is provided to selectively control one of:
the voltage of the line, reactive volt amperes caused by the load on the
line, and a power factor associated with the line.
47. The method of claim 44, wherein the appropriate capacitance is
connected only at an instant in time when an initial charge current for
the capacitance is substantially zero.
48. The method of claim 44, further comprising the steps of filtering the
potential signal to substantially reduce any harmonics of the fundamental
frequency that are present.
49. The method of claim 44, further comprising the steps of sampling the
filtered current signal for a predetermined portion of the periodic cycle
that is prior to when the potential signal reaches the predefined point in
its periodic cycle, and then determining a value for the filtered current
signal at that instant to determine the reactive power compensation.
50. The method of claim 44, further comprising the step of monitoring the
reactive power compensation provided by the capacitance to determine if
the appropriate reactive power compensation is thereby being provided.
51. The method of claim 44, further comprising the steps of detecting a
malfunction of the system and providing an alternative configuration of
capacitance to achieve the reactive power compensation should a
malfunction occur.
52. The method of claim 44, further comprising the step of maintaining the
connection of the capacitance to provide the reactive power compensation
required, even though transient distortion occurs in the voltage on the
line.
53. A method of compensating for a reactive load that draws power from a
multi-phase supply, comprising the steps of:
a. sensing a generally periodically varying electrical current on each of
the phases, producing a plurality of current signals corresponding
thereto;
b. sensing a generally periodically varying voltage on each of the phases,
producing a plurality of potential signals corresponding thereto;
c. in a manner unaffected by any harmonic distortion that may be present on
the phases, separately determining a reactive power compensation required
for each phase to compensate each phase for the reactive load connected to
that phase, wherein the reactive power compensation for each phase may be
different;
d. assembling a plurality of capacitors in arrays, each array being
associated with one of the phases, each of the capacitors in the arrays
being selectively connectable to provide at least a portion of the
reactive power compensation required for one of the phases; and
3. selectively connecting the capacitors to provide the reactive power
compensation determined for each phase.
54. The method of claim 53, wherein the reactive power compensation is
determined for each phase as a function of the current signal at a
predetermined point in a cycle of the potential signal on the phase, and
the capacitors are connected to provide the reactive power compensation
when the potential signal reaches a different point in the cycle.
55. The method of claim 53, wherein for each phase, the reactive power
compensation is determined as often as twice each cycle of the potential
signal on the phase, and the capacitors required to provide the reactive
power compensation determined are connected as often as twice each cycle.
56. The method of claim 53, further comprising the step of selectively
compensating the reactive power on each phase to control one of:
a. a volt-ampere-reactive level on each phase;
b. The voltage on each phase; and
c. a power factor associated with each phase.
57. The method of claim 53, further comprising the steps of filtering the
current signal and the potential signal for each phase, producing a
filtered current signal and a filtered potential signal, respectively,
from which any harmonics of a fundamental frequency are substantially
eliminated.
58. The method of claim 53, further including the step of monitoring the
reactive power compensation provided to producing a feedback signal
indicative thereof, for each capacitor.
59. The method of claim 58, wherein the feedback signals are used to
determine if a malfunction has occurred, and if so, further comprising the
step of providing a best available alternative configuration of the
capacitors to effect the reactive power compensation.
60. The method of claim 53, further comprising the step of protecting each
of the phase against an over voltage condition and against self-excitation
by an inductive load, in respect to the reactive power compensation
provided by the capacitors.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a first embodiment of the reactive
power compensator;
FIG. 2 is an electrical schematic diagram showing a comparator circuit that
is typical of those used in an input comparator block of the reactive
power compensator of FIG. 1;
FIG. 3 is an electrical schematic diagram of a logic debounce circuit that
is typical of two such circuits in the first embodiment;
FIG. 4A is an electrical schematic diagram of a timing logic circuit used
in the first embodiment of the reactive power compensator;
FIG. 4B is a diagram illustrating the relationship between six voltage
waveforms that are used to produce timing signals in the first embodiment;
FIG. 5 is an electrical schematic diagram of a TTL/MOS match and amplifier
(TMMA) circuit;
FIG. 6 schematically illustrates an analog amplifier circuit used to buffer
and/or amplify signals in a signal-conditioning block of the present
invention;
FIG. 7 is a graph showing several cycles of a potential signal and a cycle
of a distorted current signal, illustrating the effect of harmonic
distortion on the determination of the reactive power compensation
required for each phase;
FIG. 8 is an electrical schematic diagram of a typical dual filter circuit
used to filter out harmonic distortion;
FIG. 9 is a graph illustrating harmonic distortion in the voltage waveform
on one phase;
FIG. 10 is an electrical schematic diagram of a window circuit used in
controlling connection of the reactive power compensation capacitance;
FIG. 11 is a schematic block diagram showing a first embodiment of a switch
driver;
FIG. 12 is an electrical schematic of a triggering bridge and pulse
transformer circuit used in the switch driver of FIG. 11;
FIG. 13 is a block diagram of a compensation capacitor array for one phase;
FIG. 14 is an electrical schematic diagram of a network of optically
triggered switch drivers and solid state switches for one compensation
capacitor on one phase;
FIG. 15 is a graph illustrating control and data acquisition intervals for
several cycles of the three-phase voltage waveforms;
FIG. 16A is a flow chart illustrating a main program loop;
FIG. 16B is a flow chart illustrating the control logic used by a personal
computer (PC) in determining and controlling the connection of an
appropriate reactive power compensation capacitance to each line of a
multi-phase system;
FIG. 18C is a flow chart illustrating the details of the data acquisition
logic implemented in the main program of FIG. 16A;
FIGS. 17A, 17B, and 17C are electrical schematic block diagrams of a second
embodiment of the reactive power compensator for one phase (A.PHI.);
FIG. 18 is an electrical schematic diagram of the trigger and logic circuit
and optical driver and transmitter of the second embodiment shown in FIGS.
17A, 17B, and 17C;
FIG. 19 is an electrical schematic diagram of the watchdog timer circuit
noted in FIG. 17B;
FIG. 20A is a flow chart of the main program implemented by the controller
of the second embodiment shown in FIG. 17B; and
FIG. 20B is a flow chart of the logic implemented by the controller in
determining and controlling the connection of the appropriate reactive
power compensation capacitance for one phase.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
PC Controlled Reactive Power Compensator
Referring to FIG. 1, a first embodiment of the reactive power compensator
is shown generally at reference numeral 50. In the application of the
present invention illustrated therein, reactive power compensator 50
determines an appropriate compensation capacitor to apply to lines 52a,
52b, and 52c, which are connected to one or more inductive loads (not
shown). Lines 52 carry three-phase electrical current from a remote
generator or other three-phase source (not shown). The voltage on lines 52
is monitored by corresponding voltage transformers (VTs) 54. Similarly,
current transformers (CT's) 58 are used to monitor the electrical current
flowing from the source to each load through lines 52. Reactive power
compensator 50 separately monitors the voltage the current on each phase
and separately determines an appropriate capacitance to compensate for the
inductive load(s) connected to each phase, which may differ between the
phases. Compensation capacitors 112 are electrically connected to each
line 52 via lines 56. The capacitors can selectively be connected to
neutral or ground via solid state switching circuits 114 to provide
reactive power compensation. Reactive power compensator 50 can selectively
be operated in a manual mode in which a local or remote operator
determines the specific compensation capacitors that are connected to
provide compensation on lines 52, or it can be operated in an automatic
mode in which the reactive power compensator determines the specific
compensation capacitors that must be connected to achieve a user-selected
or predetermined volt-ampere-reactive (VAR) level on the line.
Alternatively, reactive power compensator 50 can selectively be used to
automatically maintain a predetermined voltage, or a predetermined power
factor (PF) on each line 52, or can provide reactive power compensation or
predetermined line voltage based upon a time schedule.
The mode in which reactive compensator 50 operates is selectively
determined by the operator of the device by commands entered in a local
personal computer (PC) controller 70. In addition, local PC controller 70
can optionally monitor, record, and display data corresponding to the
output signals from conventional watt, VAR, and power factor (PF)
transducer within an instrumentation bank 78, which are carried on data
leads 79. These signals are not used for determining the reactive power
compensation, but can be provided by transducers that use the same voltage
and current signals required by the reactive power compensator. For
example, as shown in FIG. 1, CTs 58a, 58b, and 58c, are connected to
monitor the electric current flowing in lines 52a, 52b, and 52c,
respectively, producing secondary current signals on leads 60a, 60b, and
60c, which are indicative of the actual current lines 52. Transducer
instrumentation bank 78 is connected to leads 60. Thus, secondary current
form CTs 58 flows through each transducer requiring a secondary current
signal indicative of electric current flowing in lines 52, and on into
leads 61 for use in determining the required reactive power compensation.
Likewise, potential signals from voltage transformers 54 are connected to
transducer instrumentation bank 78. Signals from these current and voltage
transformers are used by reactive power compensator 50 for determining the
appropriate reactive power compensation as described below.
An optional remote PC controller 72 can be connected to local PC controller
70 by a communications link 74 to control reactive power compensator 50
from the location where remote PC controller 72 is installed. For example,
electric utilities often provide a central control facility for their
distribution system from which an operator can monitor and control the
equipment connected to the distribution lines. Remote PC controller 72 may
thus comprise a relatively large computer that controls power switches and
other devices.
Reactive power compensator 50 includes a synchronization block 62 in which
timing signals are derived from the potential signals supplied by voltage
transformers 54 for us in controlling the reactive power compensator.
Signals indicative of the current and the potential on lines 52 are
amplified and filtered in a conditioning block 64. Local PC controller 70
uses the timing signals and the signals indicative of current and
potential on lines 52 to determine an appropriate reactive power
compensation for each line 52 and produces control signals that are
supplied to a plurality of triggering blocks 66. Triggering blocks 66
respond to the control signals in developing triggering signals that are
input to a plurality of switching blocks 68 and cause an appropriate
reactive power compensation capacitance to be connected to each line 52.
Only one triggering block 66 and one switching block 68 are shown in FIG.
1.
Voltage transformers 54 include primary windings 54a, 54b, and 54c,
respectively, each of which are connected between the corresponding line
52a, 52b, and 52c and ground. These voltage transformers include secondary
windings 54a', 54b', and 54c', which are, respectively, connected to leads
76a, 76b, and 76c. A signal corresponding to the voltage on each of lines
52 is thus supplied to transducer instrumentation bank 78 and to Delta-Wye
and Delta-Delta transformer banks 80. At rated voltage on lines 52, the
potential signal supplied through leads 76 is nominally 120 V AC.
Delta-Wye and Delta-Delta transformer banks 80 comprise either two
three-phase voltage transformer banks or six voltage transformers
connected in Delta-Wye and Delta-Delta configurations, producing a
relatively low voltage output signal (less than 20 V AC) that is conveyed
over leads 81 to an input comparator circuit 82 and an analog amplifier
circuit 90. The six potential signals produced by Delta-Wye and
Delta-Delta transformer banks 80 respectively correspond to the
line-to-neutral periodic sinusoidal voltage waveforms on A phase (line
52a), B phase (line 52b), and C phase (line 52c), and to the line-to-line
voltages AB, BC, and CA.
FIG. 2 illustrates a comparator 118a that is used in the input comparator
circuit 82. This circuit is typical of the six comparator circuits
supplied potential signals from Delta-Wye and Delta-Delta transformer
banks 80. As shown in FIG. 2, an input potential signal on lead 81 is
applied to two resistors 120 and 122, which are connected in series by a
conductor 121. Resistor 122 is connected to ground so that the two
resistors divide the voltage of the input signal, producing a lower level
potential signal that is applied by conductor 121 to the inverting input
of a comparator 124. The non-inverting input of comparator 124 is
connected to ground through a lead 123 and the comparator output is
connected by a lead 125 to the anode of a diode 128 and to one end of a
resistor 126. The opposite end of resistor 126 is connected to +5 V DC. A
lead 129 is connected to the cathode of diode 128 and is referenced to
ground via a resistor 130. The sinusoidal potential signal applied to the
inverting input of comparator 124 produces a corresponding square wave
ranging between -5 volts and +5 volts in amplitude. However, due to the +5
V DC level applied through resistor 126 and diode 128, the output signal
on lead 129 is a square wave ranging between approximately 0 volts and +5
volts.
Referring back to FIG. 1, the six square wave signals produced by input
comparator circuit 82 are conveyed through leads 83 to a logic debounce
circuit 84 that cleans up any jitter or noise present on their leading and
trailing edges, which can occur at each zero crossing of the potential
signal input to comparator 124. Details of a debounce circuit 132, which
is typical of the two such circuits comprising logic debounce circuit 84,
are shown in FIG. 3.
Turning now to FIG. 3, three leads 129a, 129b, and 129c, respectively carry
square wave signals (possibly having noise on their leading and trailing
edges) that correspond to the voltage waveforms on A phase, B phase, and C
phase. Thus the square waves are at +5 V DC during the positive half cycle
of the voltage waveforms and at zero during the negative half cycles. Lead
129a is connected to one of two inputs of NAND gates 134a and 134b and to
one of two inputs of OR gates 136a and 136b. Similarly, lead 129b is
connected to one input of NAND gates 134b and 134c and to one input of OR
gates 136b and 136c; and lead 129c is connected to the other input of NAND
gates 134a and 134c and to the other input of OR gates 136a and 136c.
Debounce circuit 132 also includes three D flip flops 138a, 138b, and 138c
that have their D and clock inputs grounded. Each of NAND gates 134a,
134b, and 134c are connected by leads 135a, 135b, and 135c to the inverted
present input of the three D flip flops, respectively. Likewise, the
outputs of OR gates 136a, 136b, and 136c are connected to the inverted
clear inputs of the D flip flops via leads 137a, 137b, and 137c,
respectively.
A logic level 0 applied to the inverted preset inputs of the D flip flops
causes their Q output to have a logic level 1, and a logic level 0 on the
inverted clear input causes the Q output to have a logic level 0 value.
Elimination of any jitter present on the leading and trailing edges of the
square wave signals carried on leads 129 occurs as follows. When the
incoming square wave signal on lead 129a (A phase) first rises to a logic
level 1 (at the leading edge of the square wave) the square wave signal on
lead 129c (C phase) will already be at a logic level 1. A logic level 0 is
thus output from NAND gate 134a and is applied to the inverted preset of D
flip flop 138a, causing its Q output to lock at a logic level 1 and its Q
output to lock at a logic level 0. The square wave signal on lead 129c (C
phase) then drops to a logic level 0. The first time that the square wave
signal on lead 129a drops to zero (the trailing edge of the square wave
signal) OR gate 136a will produce a logic level 0 output, causing the Q
output of D flip flop 138a to lock at logic level 0. D flip flops 138b and
138c operate in a similar manner to provide clean digital signals (square
wave signals) corresponding to one half period of the potential signal on
B phase and C phase, respectively. Although not shown in FIG. 3, a second
debounce circuit of identical design is used to eliminate jitter at the
leading and trailing edges of the square wave signals corresponding to AB,
BC, and CA potential signals.
The Q and Q logic level signals output from logic debounce circuits 84 are
conveyed through leads 140 and 141 (collectively, leads 85) to a timing
waveform logic circuit 86 (see FIG. 1), which is shown in detail in FIG.
4A. Timing waveform logic circuit 86 includes 15 NAND gates 148a through
148o, each of which have to inputs that are connected to leads 140 and 141
on which are respectively conveyed the Q and Q square wave signals output
from logic debounce circuit 84. Specifically, the signal on lead 140a is
connected to one input of NAND gate 148e, 148g, and 148l. The Q, or
inverted, version of this signal is conveyed on a lead 141a to one input
of NAND gate 148m. The signal for B phase (the Q output) is conveyed by a
lead 140b to one input of NAND gates 148d, 148f, 148h, and 148j. The
inverted form of this signal is conveyed by a lead 141b to NAND gate 148n.
For C phase, the signal on lead 140c is connected to NAND gates 148d,
148i, and 148k. The inverted form of this signal is conveyed by a lead
141c to NAND gate 148o. For AB phases, a lead 140d conveys the signal from
the debounce circuit to NAND gates 148a, 148h, and 148m. The corresponding
inverted signal is conveyed by a lead 141d to NAND gate 148l. The signal
on lead 140e for BC phases is input to NAND gates 148b, 148i, and 148n.
The inverted form of this signal is applied via a lead 141e to NAND gate
148j. A lead 140f conveys the signal for CA phase to NAND gates 148a,
148c, 148g, and 148o. The inverted form of this signal is applied through
a lead 141f to NAND gate 148k.
Leads 150a through 150o carry the signals output from NAND gates 148a
through 148o, respectively. As illustrated in FIG. 4B, these signals
comprise logic levels that change from low to high during intervals that
extend over integral numbers of 30.degree. increments, e.g., for
30.degree., 60.degree., 90.degree., etc., of a 60 Hz waveform. In FIG. 4B,
the normalized voltage waveforms corresponding to the six potential
signals are shown in respect to the inverted logic level for the signals
on leads 150a through 150o. Rather than showing the actual form of these
signals on leads 150, FIG. 4B illustrates the signals after they go
through an additional stage of inversion, which occurs in a TMMA circuit
88 (FIG. 1). These signals are used for timing, e.g., to define time
intervals around the zero crossing for each of the three-phase voltage
waveforms and around the negative peak of those waveforms. For example,
the timing signals shown in the first three lines of FIG. 4B define a
60.degree. interval centered about the point at which the voltage waveform
for each of the three phase crosses zero. The period occurring 30.degree.
prior to the zero crossing for A phase is identified as "I.sub.am1 " and
the 30.degree. interval that immediately follows is identified as
"I.sub.am2." In this notation, the "I" refers to current, "a" refers to A
phase, and "m1" and "m2" respectively refer to the first and second
periods associated with multiplexed current measurements preceding and
following the voltage waveform zero crossover point.
The timing signals on leads 150d, 150e, and 150f define 60.degree.
intervals centered around the negative peaks of each of the voltage
waveforms for the A, B, and C phases. The notation identifies these three
signals as multiplexed measurements of voltage occurring in a first and
second interval centered about the negative peak of each of the
illustrated three-phase voltage waveforms. For example, the signal on lead
150d is referred to as "V.sub.am1 +V.sub.am2' " the "V" identifying a
voltage measurement; the other terms in the notation have already been
explained.
The signals on leads 150g, 150h, and 150i represent 30.degree. intervals
occurring immediately before the negative peak for the three-phase voltage
waveforms B, C, and A, respectively. These timing signals are referred to
by notation indicating that they are used to control sampling of the
voltage on each phase. Similarly, the signal on leads 150m, 150n, and 150o
identify 30.degree. time intervals occurring 30.degree. before each of the
zero voltage crossovers for A, B, and C phases, respectively. The notation
used with each of these three signals indicates that they are used in
connection with sampling current on each of the three phases. For example,
the signal on lead 150m is identified with the notation "I.sub.as' "
wherein "I" indicates a current measurement, "a" indicates A phase, and
"s" indicates a sample and hold timing signal.
The signals on leads 150 (collectively shown in FIG. 1 as leads 87) are
input to TMMA circuit 88, details of which are shown in FIG. 5. TMMA
circuit 88 includes eight comparators 152a through 152h. Leads 150a
through 150f are connected to the inverting input of comparators 152a
through 152f, respectively. The non-inverting input of each of these
comparators is connected by a lead 164 to the common node between two
series-connected, equal value resistors 160 and 162. Resistors 160 and 162
are connected between +5 V DC and ground so that their common node to
which lead 164 is attached is at a potential of approximately 2.5 V DC.
Leads 150g through 150i are connected to the three inputs of a NAND gate
154, the output of which is connected to the non-inverting input of a
comparator 152g through a lead 156. Similarly, leads 150m through 150o are
connected to the three inputs of a NAND gate 158 that has an output
connected to the non-inverting input of a comparator 152h. The inverting
inputs of comparators 152g and 152h are connected to the 2.5 V DC
potential on lead 164.
The outputs of comparators 152a through 152h connect to leads 166a through
166h, respectively, and are thus connected to one end of pull-up resistors
168a through 168h, the opposite ends of which connect to +5 V DC.
Accordingly, comparators 152a through 152h convert the logic level signals
at their inputs, which range between 0 and 5 V DC, to signals that range
between -5 and +5 V DC. Since NAND gate 154 and NAND gate 158 invert the
logic sense of the timing signals respectively applied to their inputs,
the output of these NAND gates are connected to the non-inverting input of
comparators 152g and 152h for comparison against 2.5 V DC. The signals
output from comparators 152g and 152h are supplied on leads 166g and 166h
to a sample and hold circuit 96. As shown in FIG. 1, leads 166 comprise
leads 89.
The timing signal output on lead 166g includes three 30.degree. pulses used
to control the sample and hold of potential signals for the three phases,
i.e., to determine when to hold a value corresponding to the negative peak
voltage on each of the three phases A, B, and C. Similarly, the three
pulses comprising the timing signal output from comparator 152h on lead
166h is used to determine the time at which current is to be measured for
each of the three phases, where that time corresponds to the zero crossing
of the corresponding potential waveform for each line 52. The signals on
leads 166a through 166f are input to a multiplexer 94 (see FIG. 1) over
leads 89 to control the selection of signals indicative of current and
potential on each of the three phases A, B, and C, so that the appropriate
signal is output from multiplexer 94 to sample and hold circuit 96 via
leads 95. The timing signals applied to multiplexer 94 from TMMA circuit
88 cause the selected potential or current signal from the multiplexer's
input signals to be output from the multiplexer over leads 95 starting
30.degree. before the negative peak voltage occurs, and 30.degree. before
the zero voltage crossing of the voltage waveform, respectively. Thus, the
appropriate multiplexed potential or current signal is applied to sample
and hold circuit 96 well ahead of the point in time at which a value
sampled is to be held and supplied to local PC controller 70 over a lead
97. Details of the multiplexer and of the sample and hold circuit are not
shown, since they are generally conventional and are well known to those
of ordinary skill in this art.
Before the signals indicative of current and voltage on each of the three
phases (lines 52) are supplied to multiplexer 94 and then to sample and
hold circuit 96, as shown in FIG. 1, these signals first pass through
analog amplifier circuit 90. Signals from the analog amplifier circuit are
conveyed via leads 91 to a dynamic filter 92, which filters out harmonic
distortion.
Details of analog amplifier circuit 90 are shown in FIG. 6. The secondary
currents from current transformers 58a, 58b, and 58c flow through leads
61a, 61b, and 61c, respectively, to shunt resistors 171a, 171b, and 171c.
These shunt resistors are relatively low-valued resistors that are
connected to ground. The potential developed across the shunt resistors is
conveyed through input resistors 170a, 170b, and 170c, respectively, to
the inverting input of operations amplifiers (OP amps) 172a, 172b, and
172c; the non-inverting inputs of these OP amps are connected through
resistors 178a, 178b, and 178c to ground. Feedback resistors 176a, 176b,
and 176c determine the gain of OP amps 172a through 172c (in respect to
the resistance of input resistors 170), Op amps 172 thus produce amplified
current signals corresponding to the electrical currents on each of A, B,
and C phases (lines 52), and these current signals are carried on leads
174a, 174b, and 174c.
Similarly, leads 117a, 117b, and 117c each conduct a feedback signal
indicative of the charge current for the compensation capacitors 112
connected to the three phases, which represents the reactive power
compensation actually being provided to each phase. These feedback signals
are applied across shunt resistors 181a, 181b, and 181c, respectively. The
potential developed across these three shunt resistors are applied to the
inverting input of OP amps 182a, 182b, and 182c through input resistors
180a, 180b, and 180c, respectively. The non-inverting inputs of OP amps
182 are connected to ground through resistors 188a, 188b, and 188c.
Feedback resistors 186a, 186b, and 186c provide a desired gain. The
amplified feedback signals from OP amps 182 are carried on leads 184a,
184b, and 184c.
The three signals indicative of the voltage on A, B, and C phases developed
by voltage transformer secondary windings 54a', 54b', and 54c', are
supplied through leads 81a, 81b, and 81c to the non-inverting inputs of OP
amps 192a, 192b, and 192c through input resistors 190a, 190b, and 190c.
The non-inverting inputs of the OP amps are referenced to ground through
resistors 194a, 194b, and 194c. Feedback resistors 198a, 198b, and 198c
connect output leads 196a, 196b, and 196c of these OP amps back to their
inverting input. OP amps 192 are thus configured to buffer the potential
signals developed by the three voltage transformers connected to the
distribution lines. Leads 174, 184, and 196 collectively comprise leads
91, which convey the buffered potential signals to dynamic filter 92.
In determining the required reactive power compensation, or power factor
for each phase, the current flowing through each of lines 52 is measured
when the potential on that line crosses zero. Any harmonic distortion in
the current flowing in each of the lines 52 is reflected in the current
signals, which are output from amplifier circuit 90. Such distortion can
introduce an error in the determination of the required reactive power
compensation or power factor for each phase as shown in FIG. 7. In this
Figure, the potential and current for a single phase are shown. As will be
apparent from this illustration, the instantaneous value of current in the
distorted waveform when the potential waveform crosses zero can be
substantially greater or less than the actual value for current at the
fundamental frequency, which is represented by a dashed line in this
Figure. Distortion of the potential signal indicative of voltage on each
phase can have a similar effect on accuracy. Accordingly, it is important
to filter the signals indicative of current and potential on each phase to
eliminate the effect of harmonic distortion, so that the required reactive
power compensation can be accurately determined.
FIG. 8 illustrates part of the circuit used in dynamic filter 92 to
substantially eliminate any harmonic distortion on the current and
potential signals output from amplifier circuit 90. The signals indicative
of current flowing on A phase and B phase are conveyed via leads 174a and
174b, respectively, which are connected to input resistors 200a and 200b.
These two resistors are connected to the two inputs of a dual bandpass
filter 212, which is one of five such filters used in the dynamic filter.
The other dual bandpass filters are configured generally as shown in FIG.
8 and are used to filter the remaining current and potential signals for
each phase.
In the preferred embodiment, a Motorola.TM. type MF10CCN switched capacitor
filter integrated circuit is used for dual bandpass filter 212; however,
other filter circuits could be used for this purpose. The "Q" and other
characteristics of each section of dual bandpass filter 212 are determined
by resistor networks comprising resistors 201a/204b and 204c/204d. One end
of resistors 204a and 204b is connected to one side of resistor 200a via a
lead 202a, and their opposite ends are connected to the first section of
dual bandpass filter 212. Resistors 204c and 204d are similarly connected
to resistor 200b via lead 202b and to the second filter section. Switched
capacitance filters, such as dual bandpass filter 212, require that a
reference signal be supplied having a frequency equal to 100 times the
center frequency of the pass band. Each section of the dual bandpass
filter is therefore provided with a 6 kHz reference frequency signal over
a lead 208, preferably from a phase-locked loop circuit 206. The
phase-locked loop circuit is synchronized to the 60 Hz line frequency of
lines 52 and thus tracks line frequency fluctuations on the lines.
The filtered output signals from dual bandpass filter 212 are input to
multiplexer 94 over leads 210a and 210b; collectively, all of the output
signals from dynamic filter 92 are input to multiplexer 94 over leads 93.
Due to the use of dynamic filter 92, reactive power compensator 50 is
substantially unaffected by harmonic distortion present on the current and
potential on lines 52, and therefore, can more accurately determine the
reactive power compensation required on each phase and by monitoring the
filtered feedback signals, can determine if the compensation capacitors
112 are operating properly to provide the expected compensation.
The timing signals supplied multiplexer 94 from TMMA circuit 88 over leads
89 causes the multiplexer to select one of the filtered current and
potential signals at its input for sampling 30.degree. before that signal
is to be sampled and held by sample and hold circuit 96. In addition, the
timing signals applied to sample and hold circuit 96, also over leads 89,
define the time at which the selected multiplexed signal output by
multiplexer 94 is sampled and supplied to local PC controller 70 over
leads 97. The local PC controller includes a plug-in card (not shown) that
performs an 8-bit analog-to-digital (AD) conversion of each signal
supplied by sample and hold circuit 06, enabling a local PC controller to
compute the required reactive power compensation for each phase, based
upon the magnitude of the line current at the time the potential signal on
that phase crosses zero. Since timing signals for this operation are all
supplied by TMMA circuit 88, local PC controller 70 need only determine
the appropriate compensation capacitor(s) that must be connected to each
of the respective lines 52 to provide the computed compensation and supply
the control signals that effect connection of those selected compensation
capacitors to the lines.
Compensation capacitors 112 are only connected to compensate one of lines
52 when the potential on the line reaches a peak value. The control
signals for selecting specific compensation capacitors that are connected
at this time are supplied by local PC controller 70 over leads 101 to an
optical transmitter 100, which comprises part of triggering block 66. The
timing signals that determine when the compensation capacitors selected by
local PC controller 70 are connected to each of the lines 52 (preferably
at the negative peak voltages so that initial current flow through switch
circuits 114 is zero as the switch circuits are first turned on) are
supplied to optical transmitter 100 by the timing waveform logic circuit
86 over leads 87.
Connection of each of the compensation capacitors 112 to lines 52 is
effected as described below, using switch circuit 114, which comprises
pairs of silicon-controlled rectifiers (SCRs) and diodes, as shown
schematically in FIG. 1. Once gated into condition, the SCRs continue to
carry charge current to the capacitor so long as the direction of current
flow through the SCRs does not change. However, transient and/or harmonic
distortion in the voltage on each of lines 52 can cause the SCRs to
briefly conduct charge current to the compensation capacitors, but to
prematurely stop conducting in response to a reverse bias voltage across
the SCRs due to system voltage harmonics. This condition is illustrated in
FIG. 9. As the distorted potential signal reaches the nominal negative
peak, the SCRs are gated on to enable the flow of charge current in
respect to a compensation capacitor 112; however, when the potential
decreases from its nominal peak value, the SCRs turn off, prematurely
disconnecting the compensation capacitor prematurely from the line. To
prevent this condition, a window circuit 98 is included in triggering
block 66 for each phase, as shown in FIG. 1. Window circuit 98 receives an
inverted and filtered potential signal indicative of the voltage on one of
the phases from dynamic filter 92 through lead 93. Using this potential
signal, a "window pulse" signal is produced that insures proper operation
of switch circuits 114. (The output of window circuit 98 comprises a pulse
that is 30.degree. in width and is referred to as a window pulse signal.)
The window pulse signal starts just before the negative peak voltages on
lines 52.
The components of window circuit 98 for one of the phases are shown in FIG.
10. At the input of the window circuit, lead 93 is connected to the anode
of a diode 213, to one side of a potentiometer 219, and to a resistor 222.
The signal on lead 93 is half wave rectified by diode 213, and the cathode
of the diode is referenced to ground through resistors 214 and 216. These
resistors serve to divide the rectified voltage to an intermediate level,
which is applied through a lead 215 to the inverting input of a comparator
217. A capacitor 220, which is connected between lead 215 and ground,
filters the rectified signal, yielding a positive DC level that is
compared to the sinusoidal voltage set by the wiper of potentiometer 219
and applied to the non-inverting input of the comparator through a lead
218.
Resistor 222 is connected to a 90.degree. phase shift circuit 223 that
comprises two capacitors 223b and 233c, which are connected at each end of
a resistor 223a to ground. The 90.degree. phase shifted sinusoidal
waveform output from phase shift circuit 223 is applied to the
non-inverting input of a comparator 224; the inverting input is connected
to the wiper of a potentiometer 225, which is connected between the +5 V
DC and -5 V DC supplies (not shown). The potentiometer wiper provides an
adjustable reference voltage, to control a square wave signal on the
output of comparator 224. A lead 221 connects the output signals from
comparators 217 and 224 together, so that they are summed, producing a
logical AND signal that is applied to an output buffer 227. This summed
signal is connected to +5 V DC by a resistor 226. Potentiometer 219 is
adjusted to vary the width of the square wave signal output from
comparator 217; potentiometer 225 is adjusted to vary the start point for
the phase-shifted square wave and thus the width of signal produced by
combining the output of comparators 224 and 217. Accordingly, a 30.degree.
window signal is achieved and supplied to output buffer 227. The output
buffer comprises a comparator 227a having its output connected to +5 volts
through two series-connected resistors 227b and 227c. The voltage division
provided by these two resistors serves to convert the 30.degree. window
pulse from a signal that varies between -5 volts and +5 volts to a signal
that varies from 0 to +5 volts. The 30.degree. window pulse is output on a
lead 99, which is connected to the common node of resistors 227b and 227c.
Optical transmitter 100 comprises a plurality of light emitting diodes (not
shown) that respond to control signals from local PC controller to produce
light signals used for selecting the specific reactive power compensating
capacitors that are to be connected to each phase. These light signals are
produced by combining the timing signal provided by timing waveform logic
circuit 86 over leads 87, for each phase with the control signals from
local PC controller 70 over leads 101 and with the 30.degree. window pulse
signal from window circuit 98 over leads 99 in an AND gate (not shown).
Optical fibers 102 carrying the light signals provide complete electrical
isolation from high line voltage that could destroy the components of
reactive power compensator 50 that are electrically connected to optical
transmitter 100.
In the event that the length of optical fibers 102 required to transmit
light signals to switching block 68 causes excessive attenuation of the
light signals propagated therethrough, an optional buffer amp (not shown)
can be provided to amplify the intensity of the light signals, producing
corresponding light signals at its output that are conveyed through the
optical fibers 102 to switch driver 110 in switching block 68, as shown in
FIG. 1. Typically, the buffer amp is not required if optical fibers 102
are less than a few hundred feet in length.
FIG. 11 shows only a portion of switch driver 110 used to develop the
signals for switching a single compensation capacitor 112. The circuit
illustrated in FIG. 11 is typical of other such circuits comprising
reactive power compensator 50, each of which is used for one of the
compensation capacitors provided for connection to each line 52. Not shown
in FIG. 11 are the light sensors included within optical receiver 104 that
convert the light signals propagating throughout optical fibers 102 into
corresponding electrical pulses. The light pulse conveyed by one of
optical fibers 102 is converted to a corresponding electrical pulse by a
light sensor in optical receiver 104, and this electrical pulse is input
to an AND gate 228, the other input of which is connected to receive 10
kHz pulses produced by 10 kHz generator 108 and conveyed over leads 109.
AND gate 228 carries out a logical AND operation to combine these two
input signals, producing a 30.degree. window of 10 kHz pulses on its
output lead 229, which is connected to a triggering bridge and pulse
transformer 230.
Triggering bridge and pulse transformer circuit 230 produces n triggering
signals that are conveyed over leads 111.sub.1 through leads 111.sub.n to
a corresponding number of rectifier circuit 232, i.e., to rectifier
circuits 232.sub.1 through 232.sub.n. The number of triggering signals and
rectifier circuits needed depends upon the number of SCRs which are
required to selectively connect and disconnect each compensation capacitor
112 to provide reactive power compensation for one of lines 52, which in
turn depends on the voltage on those lines. Rectifier circuits 232 block
the negative portion of the 10 kHz pulses contained in the 30.degree.
window output from triggering bridge and pulse transformer 230, producing
a 30.degree. window that contains only positive 5 kHz triggering pulses on
leads 111.
Details of triggering bridge and pulse transformer 230 are shown in FIG.
12. Output lead 229, which is carrying the 30.degree. window of 20 kHz
pulses, is connected on both sides of the triggering bridge and pulse
transformer circuit, specifically to resistors 234a and 234b. These
resistors are connected to the gate of two field effect transistors (FETs)
236a and 236b. The drains of each of the FETS 236 are connected to ground
via lead 238. The anode of a diode 240 is also connected to ground and its
cathode is connected through a lead 242 to the collector of a PNP
transistor 244, which has an emitter connected to 170 V DC via a lead 246.
The 170 V DC is also connected to the source of FET 236a through a
resistor 248 and a resistor 250. These two resistors are connected in
series and their common node is connected to the base of PNP transistor
244. In addition, the 170 V DC is connected to the cathode of a diode 252,
the anode of which is connected to the source of FET 236b. A lead 254
connects the source of FET 236b to one end of a primary winding 258 of a
pulse transformer 256. The other end of the pulse transformer is connected
to lead 242, and thus to the collector of PNP transistor 244 and the
cathode of diode 240. A plurality of secondary windings 260.sub.1 through
260.sub.n are connected between leads 261.sub.1 /262.sub.1 through
261.sub.n /262.sub.n, respectively. Each pair of these leads comprises one
of the leads 111 thus carries an envelope of 10 kHz pulses within a
30.degree. window that are used to selectively connect one of the
compensation capacitors 112 to provide compensation for one of lines 52.
If transient or harmonic distortion in the voltage waveform reverse biases
the SCRs, prematurely stopping current flow charging compensation
capacitor 112, a successive 10 kHz pulse in the envelope immediately
retriggers the SCRs so that charge current again flows.
Operation of triggering bridge and pulse transformer 230 is relatively
straightforward. The 10 kHz pulses in the 30.degree. window, which are
applied to the gates of FETs 236a and 236b cause the FETs to conduct
current. Current flow through resistors 248 and 250 produces a bias signal
on the base of PNP transistor 244, causing it to conduct. Current thus
flows from the 170 V DC through PNP transistor 244 and through primary
winding 258 of pulse transformer 256, continuing on to ground through FET
236b. The resulting 10 kHz pulsating current flow through the primary
winding of the pulse transformer causes a corresponding current in its
secondary windings 260.sub.1 through 260.sub.n. Each time that FETs 236a
and 236b turn off, diodes 240 and 252 dissipate residual inductive current
flowing within primary winding 258.
FIG. 13 illustrates the configuration in which an array of compensation
capacitors 112.sub.1 through 112.sub.n is applied to A phase, line 52a. A
specific one of these compensation capacitors 112 is selectively connected
to provide reactive power compensation when SCRs 270.sub.1 through
270.sub.n associated with the compensation capacitor are energized to
connect that capacitor to ground. Compensation capacitors 112 are
electrically connected to lines 52, but do not provide reactive
compensation until the SCRs 270 associated therewith in each selectable
compensation block 113 are selectively gated into conduction each cycle,
providing a current path to the capacitors in respect to ground (on
neutral). As shown in this Figure, one switch driver 110 is connected to
each of the series connected SCRs 270 associated with one compensation
capacitor 112 and selectively provide the gate current required to turn on
each SCR so that the compensation capacitor can charge and provide
compensation.
It may be helpful to follow one cycle of the voltage waveform on line 52a
to understand how SCRs 270 control the compensation provided by
compensation capacitor 112. Starting with the voltage on line 52a at its
peak positive value, diodes 274 become forward biased as the voltage
beings to go negative. Current flows from ground through diodes 274,
charging compensation capacitors 112 to the negative peak value of the
voltage on line 52a. The charge remains trapped on the capacitor unless it
is connected to provide compensation during this cycle. However, if, for
example, compensation capacitor 112.sub.1 is needed to provide
compensation during this cycle, triggering signals are applied to the SCRs
270 associated with compensation capacitor 112.sub.1 as the voltage passes
through the negative peak. Diodes 274 then become reverse biased and stop
conducting, but the SCRs 270 associated with compensation capacitor
112.sub.1 are gated into conduction. These SCRs continue to conduct until
the voltage on line 52a reaches its positive peak, at which point the
current through the SCRs goes to zero and they stop conducting.
Compensation capacitors 112 for each phase can all be of nominally equal
capacitance, the compensation provided by one such device being at least
the minimum required to compensate for the smallest expected inductive
load that will be connected to one of the distribution lines 52. However,
it may be preferable to use a different nominal capacitance value for each
of the compensation capacitors 112.sub.1 through 112.sub.n in the array
for each phase, for example, providing a binary configuration wherein
their capacitance values are power of two integer multiples of some
minimum value, i.e., the sequence 1, 2, 4, 8, etc. Such choices will be
determined by the particular application of reactive power compensator 50
and can readily be accommodated by modifying the program run on local PC
controller 70.
A surge arrestor 280 is connected to line 52a through a lead 281 and is
connected to ground by a lead 282. Surge arrestor 280 is designed to
conduct in the event that the voltage on line 52 substantially exceeds the
rated voltage of the line. For example, for a 15 KV line, surge arrestor
280 would typically be designed to limit the voltage on line 52a to a
maximum of 30 KV, thereby protecting against surges caused, for example,
by lightning strikes.
Switching blocks 68 are generally similar for each compensation capacitor
112, as will be apparent from FIG. 13. A lead 56a.sub.1 connects
compensation capacitor112.sub.1 to line 52a through an inductor 278.sub.1.
Similarly, a line 56a.sub.2 connects compensation capacitor 112.sub.2 to
line 52a through inductor 278.sub.2, and likewise, each compensation
capacitor 112.sub.n in the array is connected to line 52a through inductor
278.sub.n. Associated with each SCR 270 and connected in parallel by a
lead 272 are a resistor 276 and a diode 274. In addition, a snubber
circuit comprising a resistor 286 in series with a capacitor 284 is
connected in parallel with the entire string of SCRs 270.sub.1 through
270.sub.n for each compensation capacitor. Resistor 286 and capacitor 284
protect the series string of SCRs 270 by insuring that the rated voltage
rise (dv/dt) and peak voltage are not exceeded. Inductor 278 is selected
to limit the current rise di/dt and maximum forward current to the rated
values for each of the SCRs 270. Damping provided by resistor 286,
capacitor 284, and inductor 278 limits the overshoot and peak voltage
across the SCRs.
An example may help to clarify how these components are selected for the
snubber circuit. An 8.4 microFarad capacitor 112.sub.1 is used to provide
200 KVAR of compensation in a 15 KV system. Since capacitor 112.sub.1 can
be selectively switched to provide reactive power compensation during each
cycle of operation, the string of SCRs 270 must be capable of carrying a
total of 24 amps RSM steady state current. Recalling that SCRs 270 are
preferably gated to connect capacitor 112.sub.1 to line 52.sub.a only at
the negative peak of the voltage, the string of SCRs 270 and their
associated resistors 276 and diodes 274 must be capable of blocking 24.5
KV, and SCRs 270 and diodes 274 must be capable of carrying at least 24
amps RMS current. Each SCR 270 and diode 274 typically has a voltage
rating of 1.6 KV blocking and a DC current rating of 50 amps. A safety
factor is provided by including a sufficient number of SCRs (i.e., 22) so
that the peak rating for n such SCRs is 35.2 KV peak blocking and 35 amps
RMS continuous current.
For this example, inductor 278.sub.1 is selected to provide one mH of
inductance, at a maximum rated voltage of 30 KV. This selection is based
upon the following assumptions: (1) the power line is a voltage source
with zero source resistance and 4 mH of inductance, (2) the system is
underdamped, and (3) the largest excitation is a 28 microsecond pulse of
30 KV (due to the limitation of surge arrestor 280). Accordingly, the peak
current rise di/dt (including the source inductance) is 6 amps per
microsecond. For this example, resistor 286.sub.1 should have a resistance
of 500 ohms, and capacitor 284.sub.1 should comprise a pair of 0.02
microFarad 10 KV capacitors connected in series.
It should be noted that diodes 274 can be replaced with SCRs (not shown)
having their cathodes connected in reverse bias relative to those of SCRs
270. An appropriate modification to switch driver 110 can provide trigger
signals for these additional SCRs to gate them into a conducting state at
the positive peak of the potential waveforms on each of lines 52. This
modification enable compensation capacitors 112 to be selectively
connected to lines 52 at the peak positive and/or peak negative potential
on those lines. Thus, for each phase, the required reactive power
compensation can be determined as often as twice each cycle, and the
appropriate compensation capacitors 112 connected to lines 52 twice each
cycle.
If reactive power compensator 50 is used to provide compensation at the
current and voltage levels set forth in the above example, pulse
transformers 230 must be provided with adequate dielectric insulation to
prevent short circuit flashovers. In addition, switch circuits 114 must be
properly cooled to prevent the maximum rated junction temperature of SCRs
270 and of diodes 274 from being exceeded. Immersion of pulse transformers
230 and switch circuits 114 in a transformer oil bath (not shown)
satisfies both of these requirements. The SCR.sub.s and diodes can also be
provided with cooling finds (not shown) to improve heat transfer between
these solids state device and the oil. Thermal convective currents in the
oil further improves cooling efficiency. The oil also serves as a
dielectric medium that insulates the windings of pulse transformers 230,
preventing short circuiting and arcing that would otherwise occur if the
reactive power compensator is used on relatively high voltage lines, e.g.,
rated for 15 KV. For use at much lower voltages and compensation currents,
it is possible that corrective or forced air cooling would be adequate,
and that the pulse transformer insulation would prevent arcing.
Optically Triggered Switch
Triggering bridge and pulse transformer 230, as described above, is used in
switch driver 110 to produce triggering pulses that selectively gate the
SCRs in switch circuit 114 to connect a specific compensation capacitor
112 to one of the lines 52. An optically triggered switch driver 110' can
instead be used for this purpose. One such switch driver 110.sub.1 ', for
one compensation capacitor 112.sub.1 on A phase is shown generally at
reference numeral 110' in FIG. 14. Optically triggered switch driver
110.sub.1 ' includes a plurality of optical trigger circuits 288 connected
in series, each of which control two solid state switches 350a and 350b. A
plurality, n, of these solid state switches connected in a series string
can be selectively energized to connect compensation capacitor 112.sub.1
to ground, thereby providing its compensation to line 52a. The voltage on
line 52 determines the number of solid state switches 350 that must be
used. To minimize the differential voltage across solid state switches
350, they are gated on only when the voltage on the line is at its peak
negative value, such that when initially gated on, substantially a zero
charge current flows through the solid state switches.
In the circuit shown in FIG. 14, the voltage on line 52a is nominally 5 KV
so that four pair of solid state switches 350a and 350b are required. Four
optical trigger circuits 288 are used to control the solid state switches.
As in switch driver 110, each solid state switch 350a (350b) comprises an
SCR 360a (360b) connected in parallel with a diode 362a (362b). The four
pairs of solid state switches 350 are connected in series between
capacitor 112.sub.1 and ground, and each pair of solid state switches 350
is protected against excessive peak voltage by a separate snubber circuit
368, as explained below.
A key element of optically triggered switch driver 110' is a trigger
capacitor 340, which is connected in parallel with compensation capacitor
112.sub.1. Inductor 278 is connected to both of these capacitors by a lead
348. In addition, a discharge resistor 342 is provided in parallel with
trigger capacitor 340, but is partly isolated from compensation capacitor
112.sub.1 by a diode 344, which has its cathode connected to compensation
capacitor 112.sub.1 by a lead 346 and its anode connected to lead 348 via
resistor 342. The purpose of trigger capacitor 340 is explained below.
During the negative half cycle of the potential signal on line 52a, current
flows from ground to charge trigger capacitor 340, entering each trigger
circuit 288 on a lead 338' and then on a lead 290, which is connected to
the anode of a diode 292 and a resistor 294. Diode 292 and resistor 284
are arranged in parallel; the cathode of diode 292 is connected by a lead
296 to several components, including an energy reservoir capacitor 324,
the collector of an NPN transistor 322, the anode of a light emitting
diode 298, the collector of a phototransistor 300, the cathode of a diode
302, and a primary winding 304b of a pulse transformer 306. A lead 308
connects primary winding 304b with one end of another primary winding
304a. The opposite end of primary winding 304a is connected to the cathode
of a zener diode 310 and to the collector of an NPN transistor 312. A lead
314 connects the emitter of NPN transistor 312 to a resistor 316, the
opposite side of which connects to the emitter of phototransistor 300 and
to the base of NPN transistor 312 through a lead 319. In addition, lead
314 connects to a resistor 318, the opposite end of which is connected to
the anode of a zener diode 320. The cathode of zener diode 320 is
connected to the cathode of LED 298 and its anode is connected to the base
of NPN transistor 322. The emitter of NPN transistor 322 connects to lead
324, so that its emitter and collector are in parallel with energy
reservoir capacitor 324.
Lead 314 connects to a string of parallel resistors and diodes, which
connect to a lead 338' (or a lead 338 at the top of the string).
Specifically, a resistor 326 is connected in parallel with a diode 328, a
resistor 330 in parallel with a diode 332, and a resistor 334 in parallel
with a diode 336. Diodes 328, 332, and 336 are oriented to convey positive
charge current to charge trigger capacitor 340; resistors 294, 326, 330,
and 334 are relatively high-value resistors arranged to balance the
voltage distribution across the plurality of series-connected trigger
circuits 288 in respect to the voltage across solid state switches 350.
Pulse transformer 306 includes a secondary winding 352a, which is
inductively coupled with primary winding 304a and a secondary winding 352b
inductively coupled with primary winding 304b. The anode of a diode 354a
is connected to one end of secondary winding 352a; similarly, the anode of
a diode 354b is connected to secondary winding 352b. A resistor 356a
extends between the cathode of diode 354a and a lead 365a that is
connected to the other end of secondary winding 352a. A resistor 356b is
similarly connected in respect to a lead 365b and secondary winding 352b.
A resettable circuit breaker 358 is provided between the common node at
the cathode of diode 354a and resistor 356a, and the gate of SCR 360a.
However, since only one such resettable circuit breaker is required for
each optical trigger circuit 288, the common node at the cathode of diode
354b and resistor 356b is directly connected to the gate of SCR 360b.
Each snubber circuit 368 includes a diode 372a connected in parallel with a
resistor 374a. The cathode of diode 372a and one end of resistor 374 are
connected by a lead 370 to the cathode of diode 362a in solid state switch
350a, and thus to solid state switch 350b in the above solid state switch
(if any) in the string. If at the top of the string of solid state
switches 350, lead 370 connects to reactive compensation capacitor
112.sub.1. The anode of diode 372a and the other end of resistor 374a are
connected to a resistor 376a, which is in series with a capacitor 378a.
Similarly, a diode 372b, resistor 374b, resistor 376b, and a capacitor
378b are provided for protection of SCR 360b and diode 362b. A lead 364b
connects capacitor 378b to SCR 360b and diode 362b in switch 350b.
Resistors 366a and 366b divide the blocking voltage equally across the
SCR/diode pairs (SCRs 360a)/diode 362a and SCR 360b/diode 362b). Resistors
366a and 366b also provide a discharge path for capacitor 378a around the
portion of the snubber circuit for SCR 360a and diode 362a. Likewise, a
resistor 366b provides a discharge path for capacitor 378b and is
connected across the lower portion of snubber circuit 368.
Optically triggered switch driver 110' operates in three states, including
a charging state, a holding state, and a discharge state. During the first
negative half cycle of potential on the distribution line 52 with which it
is associated, it is in the charging state. In this state, current flows
from ground through diodes 292, capacitor 324, and through diodes 328,
332, and 336 in each of the optical trigger circuits 288, to charge
trigger capacitor 340. After energy reservoir capacitor 324 is charged in
each optical trigger circuit, zener diode 320, NPN transistor 322, and
resistor 318 clamp the voltage developed across energy reservoir capacitor
324 to a relatively small value. As current flows through them, LEDs 298
emit light, indicating that optically triggered switch driver 110' is in
its charging state.
Once trigger capacitor 340 has been charged to the negative peak voltage on
line 52, the charging process stops as all of the diodes that previously
carried charging current become reversed biased. At the same time that
trigger capacitor 340 charges to the negative peak voltage on the line,
compensation capacitor 112 also charges to that voltage. Assuming that
compensation capacitor 112 is not required for compensation at this time,
optically triggered switch driver 110' enters the holding state. During
the holding state, the voltage across the series-connected string of
energy reservoir capacitors 324 and the series-connected string of solid
state switches 350 varies between zero and two times the absolute peak
voltage (i.e., Voltage=V.sub.max (1+sin (.omega.t)). It should be apparent
that when the voltage on line 52 is at the peak negative value, the
voltage at the top of both series-connected strings is zero, but when the
voltage on line 52 reaches a peak positive value, the voltage is twice the
absolute peak value, since it includes the negative peak voltage stored on
trigger capacitor 340 and compensation capacitor 112 and the peak positive
voltage of the line at that time.
Whenever a specific compensation capacitor 112 is required to compensate an
inductive load, light pulses are supplied to each phototransistor 300 in
optically triggered switch driver 110' over optical fibers 106. This light
pulse signal comprises the 30.degree. window of 5 kHz pulses. Each of the
5 kHz pulses in this light pulse envelope signal turns on phototransistors
300, thereby causing NPN transistors 312 to conduct. When NPN transistors
312 begin to conduct, the charge stored on energy reservoir capacitors 324
is conveyed through primary windings 304a and 304b of each pulse
transformer 306. These 5 kHz current pulses through the primary windings
of the pulse transformer cause a corresponding 5 kHz current to flow in
secondary windings 352a and 352b, which is applied to the gates of SCRs
360a and 360b, respectively. The gate current turns on these SCRs in each
of solid state switches 350, discharging trigger capacitor 340 through
diode 344 and enabling the selected compensation capacitor 112 to offset
the inductive load by providing a path for positive current flow to
ground.
Zener diode 310 and diode 302 comprise a "free wheeling circuit" that
carries inductive current between each of the 5 kHz pulses in the
30.degree. window. As noted above, the 30.degree. window of 5 kHz pulses
insures that harmonic distortion or transients in the voltage on line 52
cannot prematurely turn off solid state switches 350 by causing a brief
reversal in the direction of current flow through the SCRs. Diodes 354a
and 354b block reverse gate current to SCRs 360a and 360b, respectively.
A current transformer 116 is disposed at the bottom of switch circuit 114
to monitor the current between ground and compensation capacitor 112,
thereby providing a feedback signal that is supplied through lead 117 (as
shown in FIG. 1). This feedback signal is used to determine whether the
compensation capacitor is properly connected to provide compensation when
the light pulse signal is supplied to trigger its functional connection to
line 52. Current transformer 116 thus detects a fault condition if an
expected current flow to compensation capacitor 112 (i.e., reactive power
compensation) does not occur.
From the preceding discussion, it should be apparent that the time required
by local PC controller 70 for determining the appropriate reactive power
compensation capacitance for each line 52 and for controlling the
connection of the required capacitance to the line is much less than the
duration of each cycle of the voltage waveform per phase. FIG. 15 shows
several cycles of normalized voltage waveforms for each of the three
phases. Once local PC controller 70 determines the specific reactive power
compensator capacitors that should be connected to lines 52, during
interval A, a substantial time remains for data acquisition. The time
required by local PC controller 70 to determine the required reactive
power compensation is identified for each phase in this FIG. by the
intervals "A." The relatively longer remaining time identified as
intervals "B" can be used for other tasks, such as data acquisition.
During intervals B, local PC controller 70 can digitize the signals
produced by the transducers in transducer instrumentation bank 78 and can
display the instantaneous or average values indicated by those signals.
Local PC controller 70 does not directly control the times at which the
selected compensation capacitors are connected to lines 52. Its principal
function is determination of the required compensation and selection of
the specific compensation capacitors 112 that will provide the
compensation. The remaining time is spent performing data acquisition in a
main program shown in FIG. 16A. Timing signals output from timing waveform
logic circuit 86 are operative to initiate an interrupt at any time during
the data acquisition sequence. In response, local PC controller 70 breaks
out of the main program and is required to carry out the control functions
shown in FIG. 16B. FIG. 16C illustrates details of the data acquisition
function performed within the main program.
Referring initially to FIG. 16A, the main program starts at block 377 and
begins by initializing the system at block 379. During this step, the main
program clears all variables and initializes any constants used in data
conversion. The main program then enters the data acquisition routine at a
block 380, and continuously loops through this routine until an interrupt
signal is received from timing waveform logic circuit 86. The interrupt
signal causes the local PC controller to perform a control interrupt in a
block 381, so that it can carry out the control functions in a block 382,
discussed below in respect to FIG. 18B. After the control functions are
completed in block 382, local PC controller 70 returns to the point in the
data acquisition routine 380 at which it was initially interrupted.
Details of data acquisition routine 380 are shown in FIG. 16C beginning at
a start block 380a. Local PC controller 70 receives the transducer analog
signals in a block 380b. These signals are input to an A-to-D circuit
board (not shown) within the local PC controller and are converted to
digital data in a 380c. The digital data are buffered for subsequent
display and averaging by local PC controller 70 in a block 380d.
In the event that a failure flag has been set, indicating that a fault has
occurred with one or more of the compensation capacitors 112 (based on the
feedback signal provided from current transformers 116), a block 380e
causes local PC controller 70 to save the current instantaneous transducer
data in a block 380f and to display a failure indication in a block 380g.
If the component failure flag has not been set, or subsequent to display
of a failure condition, the data corresponding to the signals from each of
the transducers are averaged in a block 380b. After accumulating and
averaging a predefined number of values for each signal, an average value
for each of the data signals produced by the transducers is saved in a
block 380i. However, decision block 380i avoids saving the current
averaged value for the data prior to the predefined number of values
having been obtained. In a block 380k, the averaged (or instantaneous
data) are displayed. The program then loops back to block 380b.
The control routine performed by local PC controller 70 starts in FIG. 16B
at a block 383. In a block 384, the specific mode in which local PC
controller 70 is intended to operate is identified. Either a local
operator or a remote operator (using remote PC controller 72 connected via
communications link 74) can effect operation of reactive power compensator
50 in one of several different modes, including: (a) a VAR control mode in
which a predefined VAR level is maintained on each line 52, (b) a power
factor control mode in which a predefined power factor is maintained on
each line 52, (c) a line voltage control mode in which the voltage on
lines 52 is maintained as a predefined level, and (d) a time control mode
in which a predefined reactive power compensation is provided in
accordance with a time schedule or table (that is stored in memory)
indicating the time of day at which an inductive load connected to the
line should be compensated. The operator can also select a manual mode in
which specific compensation capacitors 112 are connected to each line.
In a block 385, the multiplexed potential signals and current transformer
signals output from sample and hold circuit 95 (see FIG. 1) are input to
the analog-to-digital conversion board within local PC controller 70.
Digital values corresponding to these signals are provided and can be
stored for display during the data acquisition routine. In a block 386,
the required compensation is determined. This determination may be made by
computing the compensation based on the reactive current measured as the
voltage on a phase crosses zero or can be determined using a look-up
table. The specific compensation capacitor(s) required for each line 52
are determined in block 386, and in a block 387, a control signal is
generated by local process controller 70 that selects the specific
compensation capacitor(s) 112 to be connected to each line 52 at the
negative peak of the potential on each of those lines. These
determinations can be made as often as twice each cycle/phase.
In a block 388, local PC controller 70 checks the feedback signals from
current transformers 116 to determine if the selected compensation
capacitors are in fact operatively connected to the lines. If a failure or
fault is detected, the failed condition is reported by setting a fail flag
in a block 390.
In the event of a fault condition, local PC controller 70 can select an
alternate configuration of compensation capacitors 112. For example,
assume that local PC controller 70 has determined that 400 KVARs of
reactive power compensation are required for A phase. However, a fault
flag indicates that one of two 200 KVAR compensation capacitors 112
previously selected has not been successfully connected to provide this
compensation to line 52a. Assume also that 50 KVAR and 100 KVAR
compensation capacitors 112 are available for use on A phase. In this
case, local PC controller 70 selects the 50 KVAR and 100 KVAR compensation
capacitors 112 in place of the failed compensation capacitor, thereby
providing the best available alternative compensation (a total of 350
KVAR). Triggering determination for the alternate capacitor configuration
occurs in a block 391 in the event that the fail flag is set in block 390.
Thereafter, in a block 392, the triggering signals used to selectively
control specific compensation capacitors for each line are transmitted.
Alternatively, if in block 389 a failure is note detected, the original
compensation capacitor configuration is triggered in block 392. Following
the completion of this control routine, the program returns to the data
acquisition routine at the same point it was interrupted.
Microcontroller Reactive Power Compensator
A second embodiment of the reactive power compensator is shown in FIGS.
17A, 17B, and 17C generally at reference numeral 50'. In reactive power
compensator 50', the potential signals on each of lines 52a, 52b, and 52c
are not used to develop timing signals as in reactive power compensator
50. Instead, the timing signals are developed from an internal timer,
which is synchronized to the potential on the line, as will be apparent
from the following explanation.
Referring first to FIG. 17A, a filter printed circuit board 400 is shown
that includes components for processing current and voltage signals for
one phase. A separate filter printed circuit board 400 is required for
each line 52a, 52b, and 52c, i.e., for each phase provided reactive power
compensation, but only one such board is shown. A signal indicative of the
potential on one phase, e.g., line 52a, is applied to a step-down
transformer 404, which reduces the level to approximately 12 V AC.
Likewise, a signal indicative of current flowing on that phase is applied
to a line current shunt resistor 406, producing a corresponding voltage
indicative of the line current. Optionally, load current can be similarly
monitored, and a signal indicative of the load current can be supplied to
a load current shunt resistor 408 to develop a potential corresponding
thereto. The feedback signal from current transformer 116, which is
connected to monitor the current flowing to the compensation capacitors
112 is applied to a capacitance current shunt resistor 410, likewise
producing a potential indicative of that current.
The signal produced by step-down transformer 404 is applied to the input of
an amplifier circuit 412, which buffers the signal. Likewise, each of the
signals indicative of current developed across the shunt resistors 406,
408, and 410 are applied to amplifiers 414, 416, and 418, respectively,
which amplify those signals to a more usable level. The signals produced
by amplifiers 412 through 418 are connected to the input of switched
capacitance filters 422, 424, 426, and 428. These filters remove most of
the harmonic distortion from the signals supplied by the amplifiers so
that generally only the fundamental 60 Hz frequency remains. using a 6 kHz
reference signal supplied by a 6 kHz generator 420 over a lead 423. The 6
kHz generator may comprise a phase-locked loop or other oscillator (not
separately shown). The filtered signals indicative of line voltage, line
current, load current, and the filtered feedback signals (from the
compensation capacitor CT) are respectively provided on leads 448, 450,
452, and 454. In addition, the signal output from amplifier circuit 412 is
applied to a 60 Hz interrupt generator 436, which produces an interrupt
signal that is output on a lead 446.
After it is filtered, a signal indicative of potential output from switch
capacitance filter 422 is applied to the input of a 30.degree. window
circuit 343 through a lead 430 and also supplied as an output on a lead
448. The 30.degree. window circuit produces a square wave on a lead 444
that is approximately 30.degree. wide and begins at about the point at
which the potential signal is at its negative peak.
As shown in FIG. 17B, reactive power compensator 50' includes a controller
442. This controller carries out all required timing functions and
processing to determine and selectively connect the required compensation
capacitor(s) 112 for a single phase. A generally conventional power supply
440 is also included to provide various voltages used by the components of
reactive power compensator 50'. Controller 442 receives the interrupt
signal from filter printed circuit board 440 via lead 446, which is
connected to the interrupt port of the controller. The interrupt signal is
generated each time that the potential signal output from amplifier
circuit 412 (in FIG. 17A) passes through zero in a positive direction and
is used to synchronize controller 442 to the voltage waveform on the phase
for which it controls reactive power compensation. That interrupt signal
is used in connection with an internal timer having a time base referenced
to the frequency of a quartz crystal 466. Quartz crystal 466 and a
capacitor 468 are connected to crystal time base inputs of controller 442
and provide a very accurate frequency reference used by it to determine
the time at which the reactive power compensation should be computed and
connected. For example, based upon the interrupt signal, controller 442
samples the signal indicative of the current in line 52 at a computed time
corresponding to the zero crossing of the potential waveform. Controller
442 determines the current based on the signal supplied on lead 450, which
is connected to one of its data ports.
Controller 442 comprises a CPU, an 8-bit analog-to-digital converter,
timer, multiplexer, and includes both random access memory (RAM) and read
only memory (ROM), Several such devices are commercially available from
different manufacturers. Using its built-in, analog-to-digital converter,
controller 442 digitizes the signals on leads 450, 452, and 454. By
comparing the line current with the load current, i.e., by comparing the
signals on leads 450 and 452 (assuming that an optional load current CT
496, as shown in FIG. 17C, is being used), controller 442 can detect a
self-excitation condition in which the load is producing current. This
condition may arise when one or more compensation capacitors 112 are
connected to provide compensation if a switch (not shown) is opened on
lines 52, allowing an inductive load to start resonantly exciting the
line. Optionally, the signal indicative of load current determined by
optional load CT 496 can be used for determining the required reactive
power compensation instead of the signal indicative of line current.
Since controller 442 also receives a signal indicative of the potential on
line 52 via lead 448, it can readily detect an over voltage resonant
condition on the line and in response, select a different reactive power
compensation (or none) to reduce the voltage. Control of line voltage
represents one of several modes noted above in which controller 442
operates. The mode of operation is set by the operator using a mode switch
470, which is connected to controller 442 by a lead 472.
A transmit receive bus 482 connects controller 442 to an optional local PC
488 via leads 484 and 486. Data supplied by controller 442 can thus be
transmitted to and displayed on local PC 488. Selection of the mode in
which controller 442 operates can also be accomplished by control signals
applied to controller 442 over leads 484 and 486 from the local PC.
Communication of control signals between local PC 488 and controller 42 is
effected over leads 490. Optionally, a remote computer 494, connected by a
data link 492 to the local PC, can supply control signals to set the mode
of operation or to define the required line voltage, power factor, or VAR
level for the connected phase. Remote computer 494 can also interrogate
the local PC to determine if a fault condition has occurred in respect to
the reactive power compensation.
A watchdog timer circuit 474 is provided to check for an output pulse from
controller 442 at least once each 20 milliseconds. In the event that the
pulse is not supplied to watchdog timer circuit 474 over a lead 476 within
this time frame, a reset signal is automatically supplied to controller 42
over a lead 480. Lead 480 is also attached to a manual reset switch 478.
Accordingly, an operator can initiate a manual reset of controller 442 by
closing manual reset switch 478.
FIG. 19 illustrates the watchdog timer circuit, which includes a NAND gate
530, that is connected to lead 476. The pulse supplied by controller 442
over lead 476 is applied to both inputs of NAND gate 530, setting its
output to a logic level 0. A capacitor 532 is connected to the output of
NAND gate 530. When the output is at a logic level 0, the opposite side of
capacitor 532 charges with current supplied from the 5 V DC supply through
a resistor 536 and a lead 534. The charge on capacitor 532 reverse biases
a diode 538, causing the inputs of a NAND gate 540, which are connected to
the anode of diode 538, to go to a logic level 0. As a result, the output
of NAND gate 540 is set to a logic level 1, which is transmitted through a
diode 542, cancelling the charge that has built up on a capacitor 546.
Capacitor 546 is connected to the cathode of diode 542 by a lead 544; the
other side of capacitor 546 connects to the +5 V DC supply. A resistor 548
is also connected to the cathode of diodes 542 controls the rate at which
capacitor 546 charges, thereby controlling the logic level applied to both
inputs of a NAND gate 550. A logic level 1 that is produced when a pulse
propagates through diode 542 to the inputs of NAND gate 550 causes a logic
level 0 to appear on its output.
The output of NAND gate 550 is connected by a lead 552 to the input of a
NAND gate 554. Assuming that a pulse was not provided by controller 442,
the output of NAND gate 550 is a logic level 1, producing a logic level 1
on the output of NAND gate 554. A resistor 560 conducts the output of NAND
gate 554 to its other input via a lead 445. A series-connected resistor
464 and a diode 562 are connected in parallel with resistor 560, the anode
of diode 562 being connected to lead 556. A capacitor 558 is connected
between ground and lead 556. A logic level 1 output on NAND gate 554 thus
charges capacitor 558 through resistor 560, until a logic level 1 appears
on both inputs of NAND gate 554, changing its output to a logic level 0.
The logic level 0 on the output of NAND gate 554 is conveyed by a lead 566
through a resistor 568 to manual reset switch 478 and is connected to the
reset port on controller 442 through lead 480. After resetting controller
442, the voltage on capacitor 558 discharges through diode 562 and
resistor 564, changing the level on lead 566 back to a logic level 1.
Referring back to FIG. 17B, a plurality of lead 458 are connected to output
ports B0 through B3 on controller 442 and convey control signals to a
trigger and logic circuit 456. In addition, the 30.degree. window signal
from the filter printed circuit board is applied to trigger and logic
circuit 456 via lead 44. In response to these control signals and the
30.degree. window signal, trigger and logic circuit 456 produces control
signals which are conveyed over leads 460 and applied to an optical driver
and transmitter 462.
Optical driver and transmitter 462 produces light signals that are conveyed
over optical fibers 464 to selectively switch the compensation capacitors
selected as appropriate for providing the required compensation to one of
lines 52. Each optical fiber 464 conveys the light signal used to trigger
solid state switches 350 to connect the selected compensation capacitor to
the line.
Details of trigger and logic circuit 456 and optical driven and transmitter
circuit 462 are shown in FIG. 18. Leads 458a, 458b, 458c, and 458d convey
control signals that are applied to one input of NOR gates 510a, 510b,
510c, and 510d, respectively, in trigger and logic circuit 456. The other
input to these NOR gates is provided by lead 444, which carries the
30.degree. window pulse signal. However, the control signals supplied over
leads 460 and the 30.degree. window pulse signal supplied on lead 444 are
inverted. As a result, NOR gates 510 have a logic level 1 output only if
both inputs are at logic level 0, i.e., when the control signal selects a
compensation capacitor during the 30.degree. window. The 30.degree. window
signal is redundant, insuring that a compensation capacitor is not
triggered to connect to line 52 other than during the 30.degree. following
the negative peak voltage of the potential waveform on that line.
The output signals from NOR gates 510 are supplied to the base of NPN
transistors 514a, 514b, 514c, and 514d through series resistors 512a,
512b, 512c, and 512d, respectively. The emitters of NPN transistors 514
are each connected to ground, and their collectors are each connected to
resistors 516 and light emitting diodes 518, which are in series. In
addition, the emitters of NPN transistors 514 are also connected to
resistors 520 and light emitting diodes 522, which are also in series, but
are in parallel with each pair of resistors 516 and diodes 518. Light
emitting diodes 518 provide a visual indication that a specific
compensation capacitor 112 has been selected, while light emitting diodes
522 transmit the light signal through optical fibers 464a, etc. required
to trigger conduction through solid state switches 350.
FIG. 17C illustrates how optical fiber 464a is connected to an optical
splitter 500 that divides the light signal propagating through optical
fiber 464a into a plurality of light signals that are supplied through
optical fibers 502 to optically triggered switch driver 110'.
Alternatively, switch driver 110 can be used in reactive power compensator
50'. The light signals supplied through optical fibers 502 simultaneously
trigger each of the SCRs in the solid state switches to conduct,
connecting the selected compensation capacitor 112 so that its
compensation is provided to line 52a, as shown in FIG. 17C. Similarly,
each of the other compensation capacitors available are selectively
controlled by optical signals propagating over other optical fibers 464 to
other optical splitters (not shown), to compensate an inductive load 498,
which is connected to line 52a. In addition, each of the two other phases,
lines 52b and 52c, are similarly separately provided an appropriate
reactive power compensation by selectively controlling the compensation
capacitance connected to those lines.
Reactive power compensator 50' has several advantages over reactive power
compensator 50. Since each phase is controlled in respect to timing
signals derived from the voltage waveform on that phase, it is not
necessary that all three phases be operational to develop timing signals,
as is the case in reactive power compensator 50. Furthermore, since the
timing is controlled in respect to an internal timer in controller 442, a
substantial simplification and reduction in the number of components
required is effected. Controller 442 also benefits from the inclusion of
watchdog timer circuit 474 to insure that it is operating properly. Since
all of the timing events are developed in software in respect to the
internal timer, controller 442 can easily be modified by changing the
program logic used to control it. Local PC 488 is not required for
determining reactive power compensation; however, it can be used for data
acquisition, if desired.
The program logic implemented by controller 442 is disclosed in FIGS. 20A
and 20B. The main program logic implemented by controller 442 starts at a
block 580. In a block 582, controller 442 initializes the system,
resetting all the variables to zero. Subsequently, in a decision block
584, the program determines if local PC 488 has provided a command, such
as a change in operating mode. If so, decision block 584 branches to a
block 586, which processes that command. Thereafter, or if a command from
local PC 488 has not been received, the main program loops back to check
again for a PC command. When the manual mode is selected, the operator can
use the local PC to select specific compensation capacitors 112 that
controller 442 connects to the line. In this mode, the controller does not
select or determine the required compensation like it does in the
automatic mode. At any time during the processing of a command from the
local PC, an interrupt can be initiated in response to the interrupt
signal input on lead 446. If an interrupt occurs, the appropriate control
action is taken in a block 590; thereafter, the controller returns to the
main loop at the point it was interrupted.
In FIG. 20B, the control logic implemented each time that an interrupt
signal occurs on lead 446 is illustrated. Following a start block 592, the
operating mode of the reactive power compensator is checked in a mode
selection block 596. A command input from local PC 488 can effect
modification of the mode selection manually determined by mode switch 470
(in FIG. 17B). Thus, for example, the operator can use local PC 488 to
select a VAR, PF, line voltage, or time schedule control mode. In a block
598, the system variables comprising voltage, line current, and the
feedback signal indicative of the charge current flowing into the reactive
compensation capacitor are input to controller 442. In a block 600,
controller 442 carries out an analog-to-digital conversion, developing
corresponding digital signals, which are used in a block 602 to determine
the required compensation, preferably using a look-up table in memory. In
a block 604, controller 442 generates the triggering command that is
output through leads 458, as shown in FIG. 17B.
Based upon the amplitude of the feedback signal indicative of charge
current supplied to the compensation capacitor (if previously connected to
provide compensation) over lead 454, controller 442 determines whether a
failure or fault has occurred in a decision block 606. If the feedback
signal fails to match an expected reactive power compensation current, a
failure is detected. If a selected compensation capacitor 112, or its
associated solid state switches 350 (shown in FIG. 17C) have failed,
controller 442 branches to a block 610, which provides the triggering
signals for a best available alternative compensation capacitor
configuration. If a failure has not occurred, or following selection of
the alternate configuration, the controller determines whether it is time
to provide the trigger signals on leads 458 in a decision block 612. As
explained above, these trigger signals are logically combined with the
30.degree. window signal on lead 444 in trigger and logic circuit 456 to
control production of the light signals that selectively activate
connection of each compensation capacitor 112 to the line). If not,
controller 442 continues to loop until it determines that sufficient time
has elapsed since the interrupt signal occurred for the potential on the
associated phase to reach the negative peak level. At that point, in a
block 614, the controller transmits triggering signals over leads 458,
which cause the appropriate reactive power compensation capacitance to be
connected to one of the lines 52.
While the present invention has been disclosed in respect to several
preferred embodiments, those of ordinary skill in the art will appreciate
that further modifications can be made thereto consistent with the claims
that follow below. Accordingly, the scope of the invention should not in
any way be limited by the description of the preferred embodiments, but
instead, should be determined entirely by reference to the claims.
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